1. Field of the Invention
The present invention relates to a bearing assembly with an absolute angle sensor built therein, which is used for detecting a rotational angle in various machines and equipments, for example, for detecting the angle necessary to control the position of a compact motor or for detecting the absolute angle in robotic articulated joints.
2. Description of the Prior Art
For the detection of the rotational angle of this type of apparatus, an encoder is largely employed. In the field of application, where more reduction in size and in cost is required, a sensor-equipped bearing having a magnetic sensor built in a bearing unit is employed. The magnetic-sensor-equipped bearing is insufficient in respect of precision as compared with an external optical encoder and, after the bearing has been incorporated in an apparatus that requires the use thereof particularly for the detection of the absolute angle, a correction work in any way such as, for example, a work for preparing a correction data while it is connected with a highly precise encoder is required essentially.
As a literary document on the sensor equipped bearing, an incremental type encoder equipped bearing, in which a multipole-magnetized magnetic generating member is provided in a rotating member, and a magnetic sensor is provided in a fixed member. (See, for example, the JP Laid-open Patent Publication No. 2001-349898, published Dec. 21, 2001.) As compared with a standard mechanism utilizing an encoder separate from a bearing, this is extremely compact in structure, but it is impossible to detect the absolute angle.
On the other hand, as a sensor capable of detecting the absolute angle, a resolver is largely known. However, even with the resolver, in order to achieve a highly precise detection of the absolute angle, some sort of correction work is necessary after the bearing has been incorporated in the equipment. By way of example, in the JP Patent No. 2607048 (JP Laid-open Patent Publication No. 7-318369, published Dec. 8, 1995), it is suggested that a highly precise incremental encoder is coupled with a direct drive motor incorporating the resolver and a correction data is prepared with the aid of a central processing unit.
In the angle detecting device of a type separate from the bearing such as the resolver, the correction data changes because of an error in shaft alignment occurring at the time of incorporation and, therefore, it is difficult to achieve a highly precise angle detection only with the resolver. Because of this, it is necessary to provide an attachment for fitting a means for performing a correction of the detected angle, for example, a high precision encoder for correction purpose to a system incorporating the resolver, for example, a direct drive motor. Also, since it is necessary to perform a correction work each time the motor is incorporated in the resolver, the user is required to undertake complicated works.
Also, the absolute angle detection with the use of the resolver is generally carried out using an RD converter. Since in principle the correction data cannot be incorporated in the RD converter itself, the example suggested in the above mentioned JP Patent No. 2607048 makes use of a correctional circuit including a central processing unit as hereinabove described and, for this reason, the angle detecting device becomes complicated and costly.
On the other hand, in the magnetic sensor equipped bearing, it is known that the absolute angle can be detected by detecting with a magnetic sensor a signal that changes over one complete rotation, for example, one cycle of sinusoidal wave per one complete rotation which signal is magnetized to a magnetic generating member. Also, since the sensor equipped bearing is of a structure, in which a sensor unit and a bearing unit are integrated together, no work such as shaft alignment is required. However, it is difficult to cause the magnetic generating member to be magnetized in a desired pattern and a highly precise angle detection has not yet been realized with the sensor itself.